In many types of display systems images are formed by pixelated optical modulators such as conventional liquid crystal displays, digital micromirror devices, liquid crystal-on-silicon modulators, etc. Although there are many advantages of these pixelated displays, they can also bring with them the disadvantage of fixed and relatively coarse addressability. Resolution relates to the number of pixels in a pixelated display panel, and addressability relates to the number of pixel locations in a display image (i.e., the display panel resolution times the number of distinct positions that each pixel can occupy in a display image).
Another problem is that each picture element in some pixelated displays includes a central imaging area or aperture that transmits or reflects image information and is bounded by an opaque border. The opaque borders can encompass significant portions of the picture elements relative to the optical apertures. In projection display systems, the projected images of these picture elements can have discernible image artifacts relating to the picture element borders. The image artifacts can include rough image edges and visible dark disruptions in image consistency.
Attempts have been made to improve the image appearance by physically shifting light from pixelated display devices in order to shift pixel images and thereby increase addressability. In one instance, a pixelated front projector shifted display pixels using a pixel-shifting device preceding or following a projection lens assembly. In one implementation, a pixel-shifting assembly included silicone material pressed between two glass plates. The assembly was positioned after a projection lens assembly and three solenoids operated together to tilt the glass plates relative to each other to effect fill-in pixel scanning. In another implementation, a cantilevered glass plate was positioned in front of a projection lens assembly and driven by a pair of modulators also to effect fill-in pixel scanning.
While providing in-fill pixel scanning both of the implementations can suffer from disadvantages relating to maintaining optimal image clarity. The pixel-shifting assembly positioned after a projection lens operates in a diverging optical space where light from the projection lens is diverging as it propagates toward a display screen. Shifting pixel locations in such a diverging optical space can introduce defects relating to differences in the propagation angles of light being projected toward different portions of the display screen. The pixel-shifting assembly positioned before a projection lens operates in a telecentric optical space in which the tilted plate causes astigmatism and reduces the lens performance.
These disadvantages could be even more greatly exaggerated if the addressability improvement methods for front projectors were employed in rear projectors. A front projector is positioned in front of a reflective display screen, together with the viewers of any displayed image. In contrast, a rear projector is positioned behind a transmissive display screen, opposite the viewers of the displayed images. Rear projectors typically have relatively short focal lengths relative to the size of the display screen, so projection lens assemblies in these projectors have much steeper projection angles of up to about 45 degrees compared to projection angles of about 25-30 degrees for projection lens assemblies in front projectors. As a consequence, addressability improvements for front projectors are likely to be substantially less successful for rear projectors.